This invention relates to a control device for a voltage type PWM inverter for reducing noises when an induction electric motor (hereinafter simply called as "induction motor") is driven by the voltage type PWM inverter at a variable speed.
Recently, there has been increasing a tendency for the induction motor to be driven at the variable speed by using the voltage type PWM inverter, and, at present, the induction motors have been used in private facilities such as airconditioning equipments of buildings. In this manner, when the induction motors are started to be used near people, electromagnetic noises of the induction motors generated by driving the inverters are highlighted.
In FIG. 5, a structure of a general voltage type PWM inverter is shown, and in FIG. 6, signal waveforms of respective parts thereof are shown. Incidentally, in FIG. 5, reference numeral 1 is a sine-wave generator, 2 is a delta-wave generator, 3 is a comparator, 4 is an inversion device, 5 is a base driving circuit, 6 is a voltage type inverter and 7 is an induction motor (IM).
More specifically, the sine-wave generator 1 generates a sine-wave control signal, as shown by (a) in FIG. 6, from an inverter frequency command f.sub.m and a voltage command V.sub.m, and the delta-wave generator 2 outputs a delta-wave modulation signal S.sub.m, as shown by (b) in FIG. 6, from a carrier frequency command f.sub.c. The comparator 3 compares (counts in a binary system) these signals, and, based on the result, a switching element of the voltage type inverter 6 is turned on and off through the base driving circuit 5 to obtain outputs as shown by (c) in FIG. 6, so that the induction motor 7 is driven at a variable speed.
As mentioned above, since a pulse-like voltage is supplied from the voltage type PWM inverter to the induction motor, a special noise which is called an electromagnetic noise is generated from the induction motor. Incidentally, it has been known that a carrier frequency component dominates when the noise is subjected to a frequency analysis. Thus, as a countermeasure thereof, it may be adopted that a switching frequency (or a carrier frequency) of PWM is raised by using a high speed switching element such as IGBT (Insulated Gate Bipolar Mode Transistor) and MOSFET (MOS-type Field Effect Transistor), so that the electromagnetic noise is moved outside an audible range. However, by doing so, there are problems such that loss of the switching element accompanying the rise of the switching frequency is increased as well as a cooling fan must be enlarged. Further, there is another problem that, due to an increase in cost, an application thereof is limited to a specific field.
Therefore, a subject of the present invention is to reduce the electromagnetic noise without enlarging the device.